Western diplomats in Riyadh said the prince's return indicate that kingdom, the which has no political parties or elected parliament, is trying to prevent a power vacuum and reassure Washington and other allies.
A day before his departure, the king reappointed several officials close to his reform course, including Saudi Arabia's relatively moderate top Islamic scholar and the ambassador to Washington.
In a rare move, Saudi Health Minister Abdullah al-Rabeeah appeared late on Sunday on state television to assure the public that the king was healthy and would return to lead the country.
Prominent journalist Jamal Khashoggi said the fourth medical bulletin in little more than a week showed the desert kingdom, known for its secrecy, wanted to dispel any rumors.
"They want to make a point that there is no room for rumors ... Everybody should know that we do have a system to resolve all unexpected situations," he added, pointing to an allegiance council set up by Abdullah to regulate the succession.
However, the princes at the top of the hierarchy are all in their 70s and 80s and the Al Saud family, which founded the kingdom with clerics in 1932, will remain a gerontocracy unless it soon promotes younger princes.
The king is thought to be 86 or 87 and Sultan is only a few years younger. Many technocrat ministers such as Oil Minister Ali al-Naimi are in their 70s.
Saudi Arabia's political stability is of global concern. It controls more than a fifth of the world's crude reserves, is a vital U.S. ally in the region, a major holder of dollar assets and home to the biggest Arab bourse.
BLOOD CLOT
Abdullah, valued by Washington as a moderate at the helm of a pivotal Muslim country, went into hospital on Friday after a blood clot complicated a slipped disc suffered the week before.
"The king will leave on Monday for the United States to complete medical tests," the Saudi Press Agency SPA said.
Health minister Rabeeah said the king "is healthy and will return to lead this proud nation." But diplomats say there has been uncertainty about Abdullah's health since he canceled a visit to France in July.
Crown Prince Sultan, who has had unspecified health problems over the past two years, was returning to Riyadh on Sunday evening from Morocco, where he had been since August.
Saudi officials say Sultan, who is also defense minister, has been working normally since returning in December from an extended medical absence. Diplomats say he was treated for cancer and has been much less active in public since.
The United States is keen to see reforms continue after the September 11 attacks of 2001 on U.S. cities brought Saudi Arabia's puritanical Wahhabi form of Sunni Islam to the top of global concerns. Fifteen of the 19 al Qaeda attackers were Saudis.
Saudi Arabia has become key to global efforts to fight al Qaeda. A Saudi intelligence tip-off helped Western governments stop package bombs destined for the United States that were sent on planes out of Yemen last month.
PRINCE NAYEF ON THE RISE
Interior Minister Prince Nayef, comparatively youthful at around 76, was appointed second deputy prime minister in 2009 in a move that analysts say will secure the leadership in the event of serious health problems afflicting the king and crown prince.
The position does not guarantee that Nayef would become king but places him in a strong position to shape policy.
In some government offices Nayef's picture has been added to that of Abdullah, Sultan and state founder Abdul-Aziz Ibn Saud.
He has also expanded his influence into areas such as economic policy. Analysts and diplomats see this as part of the jostling for position at the top of the ruling family, and most expect Prince Nayef to become king at some stage.
Last week King Abdullah transferred control of the National Guard, an elite Bedouin corps that handles domestic security, to his son Mitab, and diplomats expect more royal moves.
"This appears to be the sign that changes are coming and younger princes are now getting promoted," said Dubai political analyst Theodore Karasik.
On Sunday, SPA said the king had extended the terms of several top officials for four more years, among them the Saudi ambassador to the United States and the relative moderate grand mufti, or top Islamic scholar, Sheikh Abdulaziz Al al-Sheikh.
As things stand, only sons of the state's founder can become king. About 20 are still alive, some in ill health.
With both the king and crown prince indisposed, Prince Nayef has featured heavily in state media in the past week, overseeing the haj pilgrimage in the king's place and receiving guests.
Nayef is seen as a conservative on a range of issues. Analysts say he appears lukewarm about the social and economic reforms the king has promoted, including attempts to reduce the influence of the hardline clerical establishment in a country that imposes strict Islamic sharia law.
Another key royal, Riyadh governor Prince Salman, in his 70s, is to return home on Tuesday, SPA said.
Salman underwent spinal surgery in the United States in August and remained abroad for recuperation. He is a full brother of both Sultan and Nayef and has shown ambitions to fill top positions, diplomats say.
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